1. Field of the Invention
The present invention relates to a technique preferably applied to a position detecting device.
More particularly, the present invention relates to a technique for improving the scanning speed of an electrostatic capacity type position detecting device.
2. Description of the Related Art
There are various kinds of input devices for providing position information to a computer. Such input devices include a kind of two-dimensional position information input device called “touch panel”.
The touch panel is an input device for operating a computer and the like by a user touching a flat detection surface thereof with an input tool such as a finger, a dedicated stylus pen, and the like. In the touch panel, the position touched by the finger or the dedicated stylus pen is detected, the detected position is indicated in the screen, and an instruction is outputted to the computer.
The touch panel is widely used in a PDA (Personal Digital Assistant), ATM (Automated Teller Machine) of a bank, railway ticket-vending machine, and the like.
Various kinds of positional information detection technologies can be applied to the aforesaid touch panel. For example, there are a resistance film type position detecting device which detects the position based on a pressure change, an electrostatic capacity type position detecting device which detects the position based on a capacitance change of a film on the flat position detection surface, and the like.
An electrostatic capacity type position detecting device will be described below with reference to FIG. 6.
The position detecting device shown in FIG. 6 includes a driving section 602, a transmission selection switch 603, a reception selection switch 104, a matrix electrode 103, a preamplifier 109, an A/D converter 105, a position calculating section 106, and a synchronous clock generating section 606.
The driving section 602 generates an AC voltage having a frequency of 200 kHz, which is the frequency most easily to be absorbed by human body. The AC voltage of 200 kHz generated by the driving section 602 is selectively applied, through the transmission selection switch 603, to electrodes arranged in an X-axis direction (referred to as the “X-axis electrode group” hereinafter) 107 of the matrix electrode 103.
The matrix electrode 103 is formed by a plurality of elongated conductive electrodes arranged lengthwise and crosswise. Each of intersections of the lengthwise arranged electrodes and crosswise arranged electrodes, interposing therebetween a substantially plate-like dielectric sheet (not shown), forms a small-capacity capacitor. The AC voltage of 200 kHz is applied to these capacitors.
The reception selection switch 104 is a switch for selecting an intersection which forms a capacitor. The output of the reception selection switch 104 is supplied to the preamplifier 109, converted to digital signal by the A/D converter 105, and then inputted to the position calculating section 106.
The position calculating section 106, which is formed by a microcomputer, determines whether there is a finger on the matrix electrode 103 and calculates position information of the finger based on address information obtained from the synchronous clock generating section 606 and data corresponding to the slight signal change obtained from the A/D converter 105. Specifically, the position calculating section 106 detects a peak value after integrating the data obtained from the A/D converter 105. Further, the position calculating section 106 performs gravity center calculation based on the peak value and values before and after the peak value, and calculates the position of the finger based on the position of the obtained gravity center on a time axis.
To facilitate the following description, in the matrix electrode 103, the electrode group connected to the transmission selection switch 603 is referred to as “X-axis electrode group 107”, and the electrode group connected to the reception selection switch 104 is referred to as “Y-axis electrode group 108”.
Next, the internal structure of the driving section 602 will be described below.
The driving section 602 includes a clock generator 607, a readout section 608, a sine wave ROM 609, a D/A converter 610, a LPF 611, and a driver 612. The clock generator 607 is an oscillator for generating a clock. The clock generated by the clock generator 607 is supplied to the readout section 608.
The sine wave ROM 609 has 8 bits×256 samples of pseudo sine wave data stored therein. Based on the clock supplied by the clock generator 607, the readout section 608 designates an address of the sine wave ROM 609 and reads out the data.
The data read out from the sine wave ROM 609 by the readout section 608 is D/A converted by the D/A converter 610 and smoothed by the LPF 611, to be thereby converted into an analog sine wave signal. Thereafter, the voltage of the analog sine wave signal is amplified by the driver 612 so as to become an AC voltage to be applied to the X-axis electrode group 107.
Incidentally, the prior art of the present invention is disclosed in Japanese Unexamined Patent Application Publication No. 10-020992.